The present invention relates to a device for coupling a position measuring instrument with two objects movable relatively to one another such that the measuring instrument may be calibrated while the objects to be measured are maintained stationary.
The use of position measuring instruments to measure the relative position of two objects which are movable with respect to one another, such as a bed and slide piece of a machine tool, is well known in the art. In the case of automatic handling units, such as industrial robots, the movements are frequently measured with an angular motion transducer. Generally, the relative movement of two objects is measured by fastening a measuring scale to one of the objects and a scanning device to the other of the objects. The scanning device scans the measuring scale during relative movement to determine the relative position which may then be displayed. To protect the various components of the instrument from harmful environmental influences, the measuring instrument is provided with a casing which may be fastened to one of the objects to be measured.
For position measuring instruments that operate incrementally, the instrument must be calibrated with the aid of reference points before a measuring operation is begun or after an interruption. Such calibration processes are described in a number of patent applications, including German unexamined patent specification DE-OS 33 11 203. A special form of calibration is described in German unexamined specification De-OS 33 40 866. In this reference, an instrument for measuring the relative position of two objects is disclosed. The measuring instrument includes a graduation disk which comprises an incremental graduation and at least one reference mark. The graduation disk is borne on a shaft and housed within a casing. The graduation disk is connected with one of the objects by means of a carrier coupling and an additional shaft, also borne in the casing. The carrier coupling is such that it acts only in one rotational direction of the object. A slip coupling is used to connect the graduation disk with the object for rotation in the opposite direction. The slip coupling connects the shaft, which bears the graduation disk, with a motor which is operative to turn in the opposite direction. A scanning unit fastened to the casing scans the graduation disk. If the measurement operation is interrupted, the reference position may be reproduced by rotating the graduation disk with the pole-reversed motor. The additional shaft may be maintained in a stationary position throughout the rotation of the graduation disk. The rotation is continued until the reference mark is scanned by the scanning unit, at which point the graduation disk has been returned to the position it occupied with respect to the additional shaft at the time of the interruption. The angle of rotation, defined by the scanned reference mark and the pre-disturbance position, may then be used to recover the measurement value.
The above-described position measuring instrument, and others which are also suitable for calibrating the measuring instrument while the objects are maintained stationary, are well known in the art. However, these types of calibration instruments require increased construction and manufacturing costs over that of conventional simple position measuring instruments.
A disadvantage arises in that a vendor of position measuring instruments must be able to offer the expensive position measuring system, calibratable as described above, in addition to the standard instruments such as standard incremental angular motion transducers. The increased multiplicity of types of instruments which must be offered for sale results in an inefficient manner of operation for the vendor.
It is an object of the present invention to provide a device which may be used to connect a standard position measuring instrument with the objects to be measured such that the above-described calibration process may be performed with the objects at a standstill.